Single-wafer rapid thermal processing (RTP) of semiconductors provides a powerful and versatile technique for fabrication of very-large-scale-integrated (VLSI) and ultra-large-scale-integrated (ULSI) electronic devices. RTP combines low thermal mass photon-assisted rapid wafer heating with inert or reactive ambient semiconductor wafer processing.
Rapid thermal processing (RTP) of semiconductor wafers provides a capability for improved wafer-to-wafer process repeatability in a single-wafer lamp-heated thermal processing reactor. RTP systems mostly employ illuminators consisting of an array of tungsten halogen lamps to heat a semiconductor wafer either directly or through a graphite susceptor in a cold-wall process chamber. Conventional RTP systems typically employ illuminators that provide single-zone or very limited asymmetrical multizone control capability. Thus, an increase or decrease of power to the illuminator affects the entire wafer temperature distribution and uniformity. In general, the existing commercial RTP systems in the market can be divided into the following four categories:
(1) Top and bottom wafer heating using two arrays of linear tungsten halogen lamps. This type of RTP system configuration has traditionally dominated in the market (offered by RTP vendors such as AG Associates of U.S.A., AST of Germany; and DaiNippon Screen of Japan). In this configuration, the substrate (typically a semiconductor wafer) is placed in an atmospheric quartz chamber and is heated from both frontside and backside by two arrays of linear tungsten halogen lamps. As a result, these systems employ non-axisymmetrical illuminators, usually to heat a stationary (non-rotating) wafer.
These systems lack the essential requirements such as multi-point sensors and energy source (illuminator) axisymmetry for effective wafer temperature uniformity and repeatability control. While these RTP systems can be used for anneal (RTA or Rapid Thermal Anneal) and oxidation (RTO or Rapid Thermal Oxidation) processes, they cannot be used for Rapid Thermal Chemical-Vapor Deposition (RTCVD) applications.
In addition to the temperature uniformity and repeatability problems, these RTP system designs also suffer from localized temperature and process nonuniformities due to device or chip pattern effects. The device patterns on the wafer frontside (produced by the deposition, microlithography, and etch steps) produce different total and spectral emissivities on the wafer frontside. The regions with varying frontside emissivities (and absorptivities) absorb different amounts of photon energy or light flux from the radiative tungsten halogen lamps, resulting in different thermal responses. Moreover, the wafer frontside region with different device pattern-induced emissivities will dissipate different amounts of energy by optical radiation. This is due to the fact that the amount of radiative power loss P per unit area from the surface of an object with a temperature T (temperature in Kelvin) and a total emissivity of .epsilon. is as follows: EQU P=.epsilon..sigma.T.sup.4
where .sigma. is the Stefan Boltzmann Constant. Thus, the wafer pattern effects due to the chip patterns on the wafer frontsides can cause serious localized temperature non-uniformity problems, resulting in process nonuniformities, degradation of process repeatability, and even formation of slip dislocation.
(2) Top wafer heating using a single array of linear tungsten halogen lamps. This type of RTP system design has been introduced (for instance by AG Associates of U.S.A.) for RTCVD applications. A common configuration for this type of RTP system employs a quartz showerhead between the optical quartz window and the wafer within a cold-wall RTP process chamber. These systems may use wafer rotation through a ferrofluidic feedthrough from the chamber bottom. For this type of RTP system design, wafer rotation may somewhat alleviate the temperature and process non-uniformity problems associated with the use of non-axisymmetrical illuminator designs; it does not, however, solve the problem completely. Lack of illuminator design axisymmetry and lack of appropriate emissivity compensated multi-point pyrometry result in fundamental limitations and problems. These systems also suffer from the frontside pattern-induced temperature and process non-uniformities described earlier. Another significant problem with this type of RTP design is equipment state drift and process non-uniformity problems due to depositions on the quartz showerhead in RTCVD applications. Since the quartz showerhead is placed between the semiconductor wafer inside the process chamber and the external non-axisymmetrical illuminator used for frontside/top side wafer heating, any unwanted depositions on the quartz showerhead will affect the amount of optical power absorption by the semiconductor wafer.
(3) Two-sided tungsten-halogen lamp heating combined with a heated wafer susceptor. Some commercial RTP systems, (Applied Materials of U.S.A. and ASM of the Netherlands have introduced RTP systems based on this type of design for silicon epitaxy and polysilicon deposition (RTCVD) applications) employ a holding wafer susceptor made of a light-absorbing material such as graphite, silicon-carbide-coated graphite, or silicon carbide. These systems rely on the use of a relatively high-thermal mass heated susceptor in order to partially overcome the temperature non-uniformity and process repeatability problems. Although the use of a heated susceptor may reduce these problems, it does not completely eliminate them. Moreover, the use of a heated susceptor may introduce some new problems. For instance, in RTCVD applications, the heated susceptor can be coated by the material being deposited on the wafer (e.g., doped or undoped polysilicon; silicon dioxide; silicon nitrate; etc.). The unwanted depositions on the susceptor can result in particulate generation, cross contamination process uniformity drifts, as well as temperature measurement and process control problems. Temperature movement and control in these RTP systems is usually based on the use of pyrometry for measuring the susceptor temperature (or embedded thermocouple in the susceptor). The pyrometry measurements may experience significant errors due to the emissivity validations caused by the unwanted depositions on the susceptor. The RTP systems with this type of design may also employ wafer rotation to improve process uniformity. Again, however, as outlined in the previous RTP systems, while wafer rotation may somewhat alleviate the temperature and process non-uniformity problems associated with the use of non-axisymmetrical illuminator designs, it does not solve the problem completely. The use of a heated susceptor has another major disadvantage since it slows down the temperature control dynamics by reducing the maximum achievable heat-up and cool-down rates by a significant margin. This results in the degradation of the process throughput and an increase in cost-of ownership (COO).
(4) Hot wall RTP with a graded-zone single-wafer furnace. Eaton Thermal Processing Systems (formerly High-Temperature Engineering) of U.S.A. has introduced an RTP system based on the use of a graded-temperature resistivity-temperature hot-wall furnace. Wafer heating and cooling cycles are performed by rapidly moving a wafer between the furnace cold and hot zones. Although this type of system can provide good steady-state temperature uniformity, it lacks the means for transient dynamic uniformity control. Another disadvantage of this type of RTP system design is the need for rapid large-scale mechanical movement of the wafer in order to ramp-up or ramp-down the wafer temperatures. Moreover, this type of RTP system design can still experience localized temperature and process uniformity problems due to the device patterns effects.
In addition to the uniformity problems, conventional RTP systems also experience problems making accurate temperature measurements. Currently, the majority of RTP systems rely on pyrometric temperature measurements. Pyrometric measurements, however, have several limitations. One of the most important limitations is that the temperature measurements are dependent on the emissivity of the wafer. The emissivity is a function of various wafer states including thin films, substrate doping, and backside roughness. Emissivity is also dependent on temperature. Another important limitation of pyrometric temperature measurement in lamp heated semiconductor processing is that there is a spectral overlap between radiation from the lamps used to heat the wafer and the radiation from the wafer used to measure temperature. The pyrometer cannot distinguish between the radiation from these two sources, and thus error is introduced into temperature measurement. The error is particularly significant at low wafer temperatures during temperature ramp-up where thermal radiation from the wafer is limited but lamp radiation is significant. Lamp radiation often provides a major obstacle in making accurate measurements of temperatures below 500.degree. C. using pyrometric techniques.